Method of regenerating waste cyanid solutions.



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\VILLIAM ORR, OF SALT LAKE CITY, UTAH, ASSIGNOR TO THE GOLD AND SILVER EXTRACTION COMPANY OF AMERIOA, LTD., OF DENVER, COLO- RADO, A JOINT-STOCK COMPANY OF GREAT BRITAIN.

METHOD OF REGENERATING WASTE CYANID SOLUTEONS.

SPECIFICATION forming part of Letters Patent No. 689,017, dated December 1'7, 19101.

Application filed February 12, 1901. Serial No. 47,012. (No specimens.)

T ctZZ whom it may concern: to go to waste of no further value for the Be itknown thatI, WILLIAM ORR, a subject purposes of this art. It is besides to be noted of the Queen of Great Britain, residing in Salt that the double cyanid of zinc and potassium Lake City, in the State of Utah, have invented is also to some extent additionally developed certain newand usefulImprovementsin Methin the solution during the process of gold ex- 55 ods ofRegenerating Foulor Waste Oyanid Sotraction by the aforesaid method by reason lutions, of which the following is a full, clear, of the fact that some action takes place diconcise, and exact description, sufficient to rectly between the zinc precipitant and the enable any person skilled in the art to which free potassium cyanid in the solution, result- IO it appertains to use the same. ing in the formation of double cyanid of zinc 60 My invention relates to the regeneration of and potassium, (ZnOy 2KGy) The reactions so-called foul or waste solutions of resulting in the aforesaid formation in the cyanid resulting from the use of cyanid solusolution of this foulingcompoundthe double tions in the extraction of the precious metals cyanid of zinc and potassiummay be ex :5 from ores, whereby, as is now well underpressed as follows: stood, the said metals are dissolved out of the In the case of gold extraction by cyanid sogangue, appearing in the solution in the form lution and zinc precipitant as follows, viz: of soluble double cyanids, and are precipi- T a tated by bringing the solution containing ZAHOFKCS Bi gZ C 2o theminto contact with asuitableprecipitant- 7 as, for instance, zinc-the precipitation re- In the case of silver extraction by the same ferred to being due to the zinc or other vpreprocess as follows, viz: cipitant takin the place of the said precious r T metals in the Solution, whereby there is projgZ 2KCy+XH O duced therein instead of the said double b 2 z 75 cyanid of the precious metals in solution the In the case of the aforesaid combination of a double cyanid of zinc and potassium. The the zinc directly with the free potassium cycyanid while thus combined with the zinc in anid in the solution as follows, viz: solution remains ineffective for further sol- T c i o vent action of the precious metals, and the zn+4koyiiznzo double cyanid of zinc and potassium thus in A1103 22KC+2I0H+Hi solution is said to foul the cyanid solution, The injurious effect of this double cyanid when the same is carried from the precipitatof zinc and potassium in the solution is twoing-boxes, according to the usual economical fold. On the one hand, it renders unavailmethods, back to the ores or tailings for furable for dissolving, and consequentlyextract- 85 ther solution thereby of the precious metals. ing the precious metals, a proportion of the As the original cyanid solution is thus passed potassium cyanid (four times the weight of and repassed through the aforesaid stages inthe zinc in the solution) which would othervolved in the process of extracting the prewise be efficient for the purposes of the proc- 4o cious metals the presence of the aforesaid ess, and as the amount of this undesirable 9o undesirable double cyanid of zinc and potasdouble cyanid of zinc and potassiumincreases sium in the solution constantly and inevitably in the solution during the continuation of the increases, in proportion to the extent of preprocess the efficiency and solvent power of cipitation of the precious metals, until it bethe solution is proportionately diminished 5 comes accumulated in the solution in such until finally it ceases to be of commercial 5 quantities and the solution becomes so foul value. On the other hand, as the amount of as to result in a condition in which it is zinc in the solution increases the capacity of termed waste solutionthat is, a solution the latter for dissolving oxygen decreases, and unfit for further dissolving the precious as oxygen is essential in the solution for the metals and which heretofore has been allowed dissolving of the precious metals this reduc- 10o tion of the oxygen held in the solution correspondingly impairs its solvent efficiency. An additional and secondary evil resulting from the presence of a large quantity of the double cyanid of Zinc and potassium in the solution, together with a small quantity of free potassium cyanid in the solution, is found in the fact that when such cyanid solution is brought into contact with the precipitant, such as zinc, a granular precipitate of the single zinc cyanid (ZnOy settles out from the solution, coating the zinc precipitant and impairing its capacity for precipitating the precious metals out of the solution. Other evils arising from the presence of the double cyanid of zinc and potassium in the solution might be mentioned. It has therefore long been desired in this art to secure some method practically applicable and economical on an industrial scale whereby the solvent effi ciency of the solution might be constantly maintained and the cyanids rendered inert by combination with the zinc in the aforesaid double cyanid of zinc and potassium released from such combination and restored to their original efficiency and solvent capacity as cyanid of potassium or of other alkaline salts. It has been attempted to secure this result by precipitating'all the cyanid in the solution as-the single cyanid of zinc, then filtering off the solution, leaving the cyanid precipitate, and thereafter expelling the hydrocyanic acid by the addition of sulfuric acid and securing the resulting cyanogen in canstic-soda solution; but this method, while to some extent practicable, has proved in practice too expensive to admit of its application successfully on any industrial scale.

Wherefore the objects of my invention are, among others, to produce methods or processes whereby the cyanid of potassium, which is, as aforesaid, held up uselessly in the solution by reason of its combination with zinc,

. may be economically and rapidly recovered and restored to its original efficiency as a solvent of the precious metals,also the zinc sim ultaneously removed fro m the solution, and also the efficiency and extent of the precipitation increased. I accomplish these objects by subjecting the solution, preferably immediately after the precious metals have been removed therefrom by the zinc precipitation, to the following treatment: First, transferring the solution to any convenient and suitable tank or reservoir and retaining it temporarily there, I add thereto a certain amount of alkaline hydrate-for instance, preferably sodium hydrate, potassium hydrate, calcium hydrate, or barium hydrate, or a combination of two or more of these. The amount of such hydrate or hydrates so added will depend upon a variety of considerations-for instance, upon the extent to which it is desired to regenerate the solvent strength of the solution-that is to say, the extent to which it is desired to release from its combination with zinc the cyanid of potassium and restore it to potassium, in such a case the amount of such hydrate or hydrates added should be sufficient to secure the disassociation of a suffi cient quantity of the double cyanid of zinc and potassium to insure thereafter the complete combination with the zinc of all the sulfid added, as hereinafter described. The reaction produced by theintroduction of the aforesaid hydrate may be exemplified as followsfor instance, where sodium hydrate is used-illustrating the action of the hydrates of the monovalent alkali metals,

or where calcium hydrate is used, illustrating the action of the divalent alkaline earth metals,

In the ordinary application of my invention to mill solutions it is rarely, if ever, desirable to recover at any one time all of the held-up cyanid, but only so much thereof as is necessary, as aforesaid, to restore to the solution its normal strengththat is, the strength originally determined upon as best fitted for the treatment of the special ores for which the solution was prepared, though it is of course possible by the use of my invention to recover and restore to the solution as free cyanid of potassium all of the cyanid held up in the double cyanid of zinc and potassium. Should anysuch exceptional require- -ment arise, the amount of hydrate to be supplied can readily be determined in any convenient manner, as by the usual laboratory tests, and will depend upon the amount of double cyanid of zinc and potassium to be disassociated. I find, however, that in the ordinary practical application of my invention to mill solutions requiring regeneration to a certain point of efficiency, as aforesaid, the amount of alkaline hydrate introduced will approximate .1 per cent., or, say, two pounds per ton of solution, this proportion being of course, however, subject to increase should it be required to recover a larger percentage of free cyanid, as likewise in cases in which the solution is found to give an acid reaction. The particularhydrate or hydrates employed, as aforesaid, are selected with advantage according to the various conditions presented by different ores or tailings, bringing into the solution difierent elements and ingredients. It is therefore desirable to determine by actual laboratory tests in each instance the hydrate best adapted for the particular purpose and occasion in hand. Taking, however for instance, the case of an ordinary dilute cyanid solution which has been used in the extraction of the precious metals from ordinary average ores presenting no exceptional features, and fifty pounds per ton of the cyanid of which solution has become held up in double cyanid of zinc and potassium, and which solution contains, say, four pounds per ton only of free cyanid of potassium, I have found that on its being required to raise the strength of said solution in said cyanid of potassium to twelve pounds .per ton the result desired could be successfully accomplished by the use of my invention by the addition to the said solution of from two to three pounds per ton of alkaline hydrate, as aforesaid, preferably two pounds sodium and one pound calcium hydrate to each ton of the solution,and that the foregoing treatment in the proportions mentioned successfully produced the reactions and the extent of reactions required for the purposes of my process, as hereinafter more fully described. Having added the hydrate to the solution, as aforesaid, I next mix thoroughly, so as to insure as faras possible the disassociation of the double cyanid ofzinc and potassium into a zincate and free cyanid of the alkali. The next step in my process is to introduce into the solution, treated as aforesaid, an amount of an alkaline sullid or sulfid of one of the alkaline-earth metals, (preferably fused sodium sulfid sixty-five, per cent, Na S,) in amount somewhat less than required to precipitate all the zincate produced as aforesaid, it being important to insure that no soluble sulfid shall be left unprecipitated in the solution. The effect of the introduction of the sulfld, as aforesaid, is to precipitate the zinc as zinc sulfid,which may afterward be separated in any convenient manner,either by decantation or by filtration through a filter-press. The filtered or supernatent liquor contains the cyanid which was formerly in combination with the zinc and which now appears as free potassium cyanid rendered again available for further solution of the precious metals or other economic uses. It is of essential importance that the aforesaid hydrates should be added before the introduction of the sulfid and appreciably in advance thereof otherwise a complete reaction between the zinc salt and the soluble sulfid will be impossible and the solution correspondingly damaged rather than benefited by the introduction of the latter. The reaction in the solution resulting from -the introduction therein, as aforesaid, of the sulfid may be expressed by the following formula:

Zn(ONa),+2NaOy+2KOy+XH O-l-Na S= ZnS(pptd)+2NaOy+2KCy+4NaOH+XH Q The amount of soluble sulfid thus introduced will vary in each case, according to require mont,within the limitations above describedthat is to say, in no case should the amount of sulfid exceed that required to precipitate all the zincate in the solution-and it is important, therefore, to introduce in every instance less of the soluble sulfid than is required to precipitate all of such zincate. Taking the instances of solutions used upon ordinary ores referred to in a previous portion of this specification, and in which was introduced in the practice of my process some two or three pounds per ton of alkaline hydrate,

I obtained results entirely satisfactory by the introduction into the solution thereafter of .08 to .1 per cent. of fused sodium sulfid.

By the use of my invention as thus described I am enabled at a trifling expense and without materially disturbing the present conditions of use of solutions in cyanid-mills to recover in fouled or waste solutions as much of the held-up cyanid as is requisite to restore the solution to its original strength, and, in fact, if desired, to recover all the inert cyanid held up by thezinc.

I am aware that the use of alkaline sulfids has been suggested for the purpose of regenerating cyanid in a solution from the double cyanid of zinc and potassium; but the conditions under which such use has been attempted have proved so injurious to the solution for the aforesaid purpose of extracting the precious metals from ore, tailings, and the like as to render solutions so attempted to be treated useless for such purpose. For instance, it has in such experiments been hitherto directed that the solution shall be made strongly alkaline; but a solution of such characteris unavailable forthe purpose mentioned, because it must be repeatedly brought into contact with zinc in order to precipitate the precious metals, and owing to such alkalinity will attack the zinc, causing a prohibitory waste thereof and the production of zincates which foul the solution disastrously. Again, the use in this connection of alkaline sulfids in excess-even a slight excess-as formerly directed is equally fatal, because it results, as I have found, in the removal of oxygen, the latter being indispensable to the solution of the precious metals. Other equally fatal obj ec-tions to uses of alkaline sulfids, as hitherto suggested in this connection, might be pointed out, and it will be understood that I do not claim, broadly, every such use.

Having thus described my invention, what I claim as new, and desire to secure by Letters Patent, is the following, viz;

1. The method of regenerating, for subsequent use in the extraction of precious metals by processes employing a zinc precipitant, cyanid solutions fouled or reduced to waste by the presence of the double cyanids of zinc and potassium, which consists in first introducing into such solution alkaline hydrate in quantities sufficient to release, from its com bination with the zinc in solution, the amount of cyanid of potassium required to be restored as free cyanid and to secure the disassocia' tion of a sufficient quantity of said double cyanid to insure thereafter the complete com bination with said last-mentioned zinc of all the sulfid added as hereinafter described, next agitating the solution until the hydrate has been brought into contact with said double cyanid resulting in a zincate and free cyanid of the alkali, next, introducinginto the solution soluble alkaline sulfid in amount less than required to precipitate all said zincate,

next, further agitating the solution until said alkaline sulfid and finally removing the resulting zinc-sullid precipitate, substantially as and for the purposes described.

3. The method of regenerating cyanid solutions which have become fouled owing to the presence of double cyanid of zinc and potassium which consists first in introducing into such solution alkaline hydrate, being hydrate of the monovalent alkali metals and hydrate of the divalent alkali-earth metals in the proportion of about two of the hydrate of the monovalent alkali metals to one of the hydrate of the divalent alkali-earth metals, next, after the disassociation of the said double cyanid by the hydrate, in introducing into the solution soluble alkaline sulfid and finally removing the resulting zinc-sulfid precipitate, substantially as and for the purposes described.

WILLIAM ORR.

\Vitnesses:

WALTER D. EDMONDS, PHILIP 0. PEoK. 

